1. Field of the Invention
The present invention relates to a mask ROM and its fabrication method and, more particularly, to a mask ROM capable of continuously coding data by implanting ions, using a mask of a user during the fabricating process.
2. Discussion of Related Art
A ROM is a nonvolatile memory device in which stored data are not changed in a normal operation state. A ROM is classified according to the methods for storing data into the ROM. There are a mask read only memory (ROM), a programmable ROM (PROM), an electrically programmable ROM (EPROM) and an erasable and electrically programmable ROM (EEPROM).
The mask ROM is coded with its data, i.e., has its data stored in it, by using a specialized mask (representing particular required for a user) during the fabrication process. Data stored in a mask ROM is not able to be changed, rather it is only possible to read the data. A type of mask ROM causes a predetermined transistor have a status that differs from other transistors by implanting impurity ions, so that a datum is coded. That is, the mask ROM causes a predetermined transistor to have an OFF state by implanting impurity ions during fabrication. Transistors for which impurity ions are not implanted during fabrication have an ON state, and vice versa. Therefore, the data are coded.
As illustrated in FIG. 1, a conventional mask ROM has a buried oxide layer 19 which is perpendicular to a wordline of gate 23. A high concentration impurity region (not shown) made of a common source and drain region and used for a bit line is formed under the buried oxide layer 19, so that the word line is perpendicular to the bit line. Accordingly, the word line and the bit line intersect and form transistors. First and second transistor channels 27 and 29 are formed between the impurity regions under the buried oxide layer 19, overlapping with the gate 23. The transistor T1 having the first channel 27 that is coated with P conductivity type material maintains the OFF state, and the other transistor T2 having the uncoated second channel 29 is not programmed and maintains the ON state.
As illustrated in FIG. 2, a gate oxide layer 17 and a buried oxide layer 19 are formed on a P type semiconductor substrate 11. The buried oxide layer 19 is thicker than the gate oxide layer 17. An impurity region 21 having N type impurity ions is formed under the buried oxide layer 19. The impurity region 21 is the common source/drain of the transistors and used for a bit line. A gate 23, perpendicular to the impurity region 21, is formed on the gate oxide layer 17. A portion of the semiconductor substrate 11, positioned opposite to the gate 23 becomes the first and second channels 27 and 29. The transistor T1 having the first channel 27 is made of the P type impurity ions and maintains the OFF state. The transistor T2 having the second channel 29 maintains the ON state.
A process for forming the above-described conventional mask ROM will now be described with reference to FIGS. 3A-3D.
As illustrated in FIG. 3A, a first photosensitive layer 13 is deposited on the semiconductor substrate 11 made of P type silicon. The photosensitive layer 13 is exposed to light, developed and patterned to expose selected portions of the semiconductor substrate 11. An N type impurity ion such as As or P is heavily doped in the semiconductor substrate 11, using the first photosensitive layer 13 as a mask, to form an ion implanted region 15.
As illustrated in FIG. 3B, the first photosensitive layer 13 is eliminated. The surface of the semiconductor substrate 11 is implanted with impurities during a thermal oxidation process, and the gate oxide layer 17 is formed on a portion where ions are not implanted. The rate of oxidation in the portion of the semiconductor substrate 11 where the ion implanted region 15 is formed is 15 to 20 times faster than that of the portion where ions are not implanted due to a lattice damage, enabling the formation of a thick buried oxide layer 19. During thermal oxidation, impurity ions in the ion implanted region 15 are activated, so that they function as the common source and drain region. The impurity region 21 is used for the bit line.
As illustrated in FIG. 3C, impurity ions such as polycrystal silicon are doped on the gate oxide layer 17 and the buried oxide layer 19 using chemical vapor deposition (CVD). They are patterned to be perpendicular to the impurity region 21 in a photolithography method, effectively forming a gate 23. Therefore, there is formed the transistor whose channel is the portion corresponding to the gate 23 between the impurity regions 21 of the semiconductor substrate 11. The second photosensitive layer 24 is deposited on the overall surface of the substrate, exposed to light, developed and patterned to expose the predetermined transistor. Impurity ions such as B or BF.sub.2 are heavily implanted in the substrate to form the ion implanted region 25, using the second photosensitive layer 24 as a mask.
As illustrated in FIG. 3D, the second photosensitive layer 24 is eliminated. The impurity ions in the ion implanted region 25 are heat-treated and diffused to form the first channel 27, where P type impurity ions are heavily doped. The channel where the P type impurity ions are not doped becomes the second channel 29. The transistor T1 that is used for the first channel 27 is coded, and the other transistor T2 that is used for the second channel 29 is not coded.
The above-described conventional mask ROM has a low integration because the impurity region used for the source and drain regions and the gate are formed on the same surface.